I'll second the EPDM roofing material idea. For the first 10 years, my
Minnesota pond (10' by 15' by 3') was lined with the thinner vinyl and
I was constantly fighting leaks in the last few years. I get 12 to 18
inches of ice on it each year and that's a tough environment for a
liner. The UV rays take a toll too. Two years ago I put in the heavy
roofing material and the fish or plants don't seem to mind it. I
expect it to last much longer. Ponds are not maintenance free, but I
really like the sound of the waterfalls and the light reflecting off
the water. Even the neighbors like it, although it shuts off at night.

There is a news group named rec.ponds.moderated that is a good source
for specific questions and information.

Bigger and thicker is better.

dss